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Essay on Red Blood Corpuscles (RBC)
Essay Contents:
- Essay on the Introduction to Red Blood Corpuscles
- Essay on the Functions of Red Blood Corpuscles
- Essay on the Composition of Red Blood Corpuscles
- Essay on Normal Red Blood Corpuscles Count
- Essay on the Abnormal Forms of Red Blood Corpuscles
- Essay on the Development of Red Blood Corpuscles
- Essay on the Energy Metabolism of Red Blood Corpuscles
- Essay on the Life Span of Red Blood Corpuscles
- Essay on the Fate of Red Blood Corpuscles
- Essay on the Red Blood Corpuscles and Haemoglobin
Essay # 1. Introduction to Red Blood Corpuscles:
The mature human erythrocyte is a circular, biconcave, non-nucleated disc. The edges are rounded and thicker than the centre. Hence, the central portion appears to have a lighter shade. When viewed from the side it looks like a dumb-bell.
In all mammals, excepting camels, the red blood corpuscles are of this type. In the camels the shape is oval; otherwise they are same as in others. In the non-mammalian vertebrates the red cells are oval, biconvex and nucleated. During the early part of foetal life even the mammalian red cells are all nucleated. But in the later part the nucleated cells disappear from the circulation.
The mature red cell is soft and flexible and can readily squeeze through narrow capillaries. Inside the corpuscles there is a frame work, chiefly composed of proteins and lipids. The meshes of this framework remain filled up with haemoglobin. Under the microscope a single red cells seems to have a light brown or yellowish colour. But when seen in bulk the red blood corpuscles appear to be red.
Histologically, no definite cell membrane has been demonstrated, but still, there seems to be a delicate outer envelope formed by the condensation of surface molecules. It is believed that this membrane is composed of proteins, phosphatides and cholesterol.
The inner and outer layers are made up of proteins and the middle layer of lipids. The permeability of this membrane is highly selective. The bigger colloidal molecules as well as the cations (K, Na, etc.) are not allowed to pass. But certain crystalloids (urea, etc.) and the anions (CI, -HCO3, etc.) are freely permeable.
Essay # 2. Functions of Red Blood Corpuscles:
i. Respiratory:
Red blood corpuscles carry oxygen and carbon dioxide.
ii. Acid-Base Balance:
They help to maintain acid-base balance. It is carried out by the buffering action of haemoglobin and other intracellular buffers.
iii. Red Cells Maintain Ion Balance:
By the special permeability of the cell membrane, the red cells help to maintain balance of positive and negative ions in the blood.
iv. Viscosity of Blood:
Red cells help to maintain the viscosity of blood.
v. Pigments:
Various pigments are derived from haemoglobin after the disintegration of the red cells, e.g., bilirubin, biliverdin, etc.
Essay # 3. Composition of the Red Blood Corpuscles:
Each cell is composed of a colourless envelope enclosing semiliquid material, 65% water and 35% solids of which 33% is haemoglobin bound to 2% stromal meshwork of protein, phospholipid, cholesterol, cholesterol esters and neutral fat. Other organic substances, such as urea, amino acids, creatinine, adenyl pyrophosphates, diphosphoglycerates, etc., are also present, but in very small amounts.
Of the total lipids 60% is phospholipid (half of this is cephalin), 30% free cholesterol and 10% fats and cholesterol esters. Of the salts in the corpuscles, potassium phosphate is the chief. [It should be noted that the chief salt of plasma is sodium chloride.]
Essay # 4. Normal Red Blood Corpuscles Count:
The normal average count in adult male is taken as 5 million and in female 4.5 million per cubic millimetre. But most observers agree that the actual figures are a little higher, 5.4 million in males and 4.8 million in females. In infants the count is 6 to 7 million, whereas in foetus 7.8 million. In the first ten days of the postnatal life large numbers of red cells are destroyed. [This is one cause of Jaundice in the new-born.]
Variations of Red Cell Count under Various Physiological Conditions:
i. Diurnal Variation:
Variations amounting to about 5 percent occur in twenty-four hours. The count is lowest during sleep, then gradually rises and becomes maximum in the evening.
ii. Muscular Exercise:
Exercise raises the count temporarily.
iii. Altitude:
At higher altitude the count rises, whereas at lower altitude (i.e., high barometric pressure) the count falls.
iv. High External Temperature:
High external temperature Increases the red cell count.
v. Any condition which lowers the oxygen tension of arterial blood increases the red cell count.
vi. Injection of Adrenaline and Excitement:
Injection of adrenaline and excitement increase the count.
vii. Size, Volume, Thickness, etc., of Red Blood Corpuscles:
The diameter of the red blood corpuscles when in the body varies from 5.5µ to 8.8µ and about one-third of this in thickness. But in the dried and fixed films (as done for clinical purposes) the cell shrinks to some extent and has a mean diameter 7.2µ. (Fig. 4.5 A and B).
The average size of the red blood corpuscles can be determined from a film preparation with the help of an instrument known as Halometer. It can also be measured directly under the microscope by an instrument known as Micrometer. The average thickness of a red cell is about 2.2 µ (Fig. 4.5-C), the average surface area is about 120 square µ and the average volume is about 87 cubic µ.
Essay # 5. Abnormal Forms of Red Blood Corpuscles:
A variation in size is known as anisocytosis. Red cells that are larger than normal are macrocytes, those smaller microcytes. Deviation from normal shape is poikilocytosis. The small size and the great number of the red blood corpuscles are of considerable importance. This makes the available surface area very large and thereby facilitates rapid exchange of gases and other materials between the cells and the plasma.
It is estimated that the total surface area of the red cells is about 1,500 times greater than the surface area of the whole body. Per litre of blood, the total surface area of the red cells is about 600 sq. metres. The diameter and the volume of red cell increase when blood tends to become acid. Hence, increased CO2 tension, anoxia, acidosis, etc., increase the volume and diameter of the red cells.
Due to these reasons the red cells in the venous blood are slightly larger than those in the arterial blood. Alkalosis produces opposite effects. The absence of nucleus is of great benefit. It gives the red blood corpuscles their biconcave shape and also makes room for more haemoglobin. The biconcave shape of the red cells is also of great advantage for many reasons.
For instance:
(a) It allows considerable alteration of the cell volume without increasing the tension on the cell membrane. The concave part freely moves out and in as the volume increases or diminishes and in this way, can withstand considerable change of osmotic pressure and resist haemolysis.
In the venous blood about 7.5% increase of the cell volume occurs. This is due to the shift of the CI ions (vide under Chloride Shift) into the cells, increasing the osmotic pressure and consequently drawing more water into the cells, and
(b) It allows easy ‘folding’ of the red cell when the latter passes through capillaries whose diameter is often narrower than its own.
(c) Due to biconcave shape the haemoglobin remains distributed in a very thin layer. This facilitates quick saturation or desaturation with the gases.
Essay # 6. Development of Red Blood Corpuscles:
Theories of Origin:
There are two theories; intravascular and extravascular. Formerly, it was believed that the red blood corpuscles were formed only intravascularly from the capillary endothelium. But Turnbull and Gilmour (1941) have shown that they are undoubtedly produced from the extravascular sources.
They have shown that the parent cell is an extra- vascular cell, known as haemocytoblast which by active amoeboid movement burrows into the blood sinuses, multiply there and mature into normal erythrocytes. The general trend of opinion seems to be in favour of the extravascular theory now.
Site of Development:
In the embryo, the red cells develop from the area vasculosa of the yolk sac. The mesodermal cells in this area remain as a mass of protoplasm with scattered nuclei. Fluid droplets appear in it, and run together to form channels.
This fluid is the primitive plasma and those cells which line these channels become the vascular endothelium, from which the early red cells develop. At first the cells are all nucleated. From the middle of foetal life the nucleated cells disappear from the peripheral circulation.
Stages of Blood Formation in the Embryo and Foetus:
There are three successive stages of blood formation in the embryo and foetus:
1. Mesoblastic Haemopoiesis:
Mesoblastic haemopoiesis is first demonstrable in the first two months of embryonic life. Throughout this period, no blood forming organ is present and most other cells are formed outside the embryo. This stage is markedly diminished in a human embryo of nine weeks.
2. Hepatic Haemopoiesis:
Hepatic haemopoiesis constitutes the second stage and includes the splenic and thymic blood formation. This stage occurs from the second to the fifth month.
3. Myeloid Period of Haemopoiesis:
The final or myeloid period of haemopoiesis begins approximately at the fifth month, with the establishment of the placental circulation. At first, the liver is chiefly occupied with erythropoiesis and the bone marrow leucopoiesis, but the bone marrow soon takes overall haemopoietic activity. The other sites however retain their haemopoietic potentialities throughout life.
After birth the bone marrow is the main site of erythrogenesis. During early years all bones are filled up with blood forming red marrow, but by twentieth year almost all the long bones are replaced with inactive yellow marrow and red blood corpuscles formation in this location stops. Only the upper ends of femur and humerus contain red marrow and continue to form red cells throughout life. In addition to this, the vertebrae, the ribs and the flat bones produce red cells continuously.
Arterial O2 Content, Tissue O2 Tension and Erythropoiesis:
The most important factor controlling the rate of red blood corpuscles production is the oxygen content of the arterial blood, a decrease in oxygen content stimulates erythropoiesis. The oxygen content of the blood may fall either due to diminution of the amount of haemoglobin content of blood or due to inadequate oxygenation of haemoglobin.
During haemorrhage there is fall in circulating haemoglobin which leads to increased production of reticulocytes. In high altitude also there is increased red cell production. Decrease in oxygen content in the arterial blood leads to decrease of oxygen tension in the tissue. It has been suggested that lowering of oxygen tension in the tissues has got no direct stimulating effect but acts through humoral mechanism.
It is the erythrocyte-stimulating factor or erythropoietin or haemopoietin which stimulates erythropoiesis. Erythropoietin is a glycoprotein of low molecular weight. It is formed in the renal tissue probably due to the effect of adrenocorticotrophic hormone (ACTH) or some other hormones of the anterior lobe of the pituitary.
Maturation and Multiplication:
It should be noted that the phenomenon of development involves two distinct processes-one is multiplication and the other is maturation. By the latter process, the cell becomes specialised to perform that particular work for which it is meant. It should also be noted that, these two distinct multiplication and maturation are antagonistic attributes. They cannot go hand in hand in the same proportion. Maturation cannot take place when multiplication is actively proceeding and multiplication will cease in the same ratio as maturation is in progress.
In the case of red blood corpuscles the process of maturation involves three different changes:
1. First, a gradual reduction of cell size;
2. Secondly, the acquirement of haemoglobin; and
3. Thirdly, the disappearance of the nucleus.
Of these three, again, haemoglobin formation seems to be the most important. For this reason it will be seen that as soon as haemoglobin begins to appear, cell division gradually ceases.
Stages of Development (Fig. 4.6):
The different stages of development, as suggested by both intravascular and extravascular theories, are summarised below. The names used by the intravascular school are given in brackets. The intravascular theory suggests that the first stage starts with the endothelial cell, while the extravascular theory holds that it starts with haemocytoblast. Histologically these cells differ. In all the other stages the histology of the corresponding cells is closely similar.
The stages are as follows:
1. Haemocytoblast:
A big cell, 18-23 µ in diameter, with a large nucleus and a thin rim of deep basophilic cytoplasm. [According to intravascular theory, this stage starts with endothelial cells. They are large, undifferentiated reticulo-endothelial cells, lining the sinusoids of bone marrow. They proliferate and give rise to megaloblast.]
2. Proerythroblast:
14-19 µ in diameter, basophilic cytoplasm, large nucleus with distinct nucleoli and a reticulum of the chromotin threads. Haemoglobin absent. Actively multiplies into the next form, only in states of stress.
3. Early Normoblast:
Smaller in size, 11-17µ, nucleus and chromatin more dense. Nucleoli absent or rudimentary. Actively divides and passes into the next stage.
4. Intermediate Normoblast:
Size still smaller, 10-14 µ, with fewer mitochondria. Nucleus more condensed, of ten eccentric. No nucleolus. Haemoglobin appears at this stage and consequently the cytoplasm becomes polychromatic. The later forms do not divide; they mature to form late normoblasts. The process of maturation involves the acquisition of more haemoglobin and a condensation of the nuclear chromatin.
5. Late Normoblast:
The size is more reduced; so that it is just a little larger (7-10 µ) than a mature red cell. The (Normoblast) nucleus is very dense and takes a deep stain (pyknotic), looking like a drop of ink (ink-spot nucleus).
The amount of haemoglobin has increased. The further maturation of the normoblast involves the complete loss of the nucleus.
There are two views:
(a) Nucleus undergoes fragmentation (karyolysis). The amount of haemoglobin increases at the expense of the nucleus,
(b) Nucleus is extruded from the cell as a whole.
The factors that cause dissolution of the nucleus are not at all understood. The normoblast, after the loss of the nucleus, passes into the next stage.
6. Reticulocyte:
When stained with vital stain (such as cresyl blue), these cells show a net-like structure (reticulum) in the cytoplasm. From the reticular appearance its name has been derived. In normal blood they are present to about 1%; in the new-born baby, about 30-50%. In the first week of life the number drops to 1%. It is from this stage that the red blood corpuscles begin to appear in the peripheral circulation.
Their number increases when active regeneration of red cells takes place, for instance during recovery from anaemia. Under this condition even normoblasts may be found in the peripheral circulation. Regarding the origin of the reticulum, it is held that they are the remnants of the original basophilic cytoplasm of the immature non-haemoglobinised red cells.
Instead of using vital stains, if ordinary Leishman’s stain is used, the reticulocytes may appear in two other forms. They may either take a diffuse blue stain (polychromatophilia) or may display a number of discrete blue particles in the cytoplasm (punctate basophilia). The latter variety is especially prominent in cases of lead poisoning for no obvious reasons. The reticulocyte matures into erythrocyte.
7. Normal Erythrocyte:
The normal red cell.
It takes about 7 days-time to pass from the stage of proerythroblast (megaloblast) to that of reticulocyte and another two days from reticulocyte to mature erythrocyte.
Factors Controlling Erythropoiesis:
The red cells are constantly being destroyed and are regenerated. The rate of destruction and regeneration are same, otherwise, a constant red cell count would not be possible. These facts signify that some stimulus, exactly proportional to the number of red blood corpuscles destroyed, is constantly acting upon bone marrow in order to replace the lost cells. The exact nature of the stimulus is not known.
Certain factors are necessary for the formation and maturation of red cells.
They are as follows:
i. Diet:
Food, rich in first class proteins (or proteins of high biological value), is important. First class proteins supply essential amino acids for the synthesis of globin of haemoglobin. It is also necessary for the formation of stromaproteins and the nucleoproteins of the red cells.
ii. Anoxia and Erythropoietin or Erythrocyte-Stimulating factor (ESP):
The exact nature, as already mentioned, is not known. But the stimulus becomes more effective or is supplemented when there is low O2 tension in the tissues. When air with low oxygen tension (as in high altitude) is breathed for some length of time, the red blood corpuscles count rises due to liberation of erythropoietin or haemopoietin or erythrocyte-stimulating factor (ESF). It stimulates the bone marrow and increases the rate and maturation of red cell formation.
iii. Stimulus for Maturation:
It is now generally agreed that as the red blood corpuscles mature, various factors influence the passage of the maturing red cell from stage to stage.
Our knowledge in this respect though far from complete, may be briefly summarized as follows:
Haemocytoblast:
Nothing definite is known as to the factors that come into operation in this stage. In certain (Endothelial diseases this stage fails to occur. Red cell formation stops and the result are known as aplastic anaemia.
Proerythroblast:
Haematinic principle of Castle (Haemopoietic principle or PA factor) vitamin B12 (extrinsic factors) and folic acid are required for the conversion of proerythroblast (megaloblast) into early normoblast (erythroblast). For the proper absorption of extrinsic factor, intrinsic factor present in gastric mucosa is essential. For details vide anaemia.
Early Normoblast:
A number of factors influence this process.
1. Metals:
i. Iron:
Essential for haemoglobin formation especially for synthesis of haem. Dietary intake of iron is required for the formation of haemoglobin. Deficiency of iron in the diet leads to Hypochromic or Iron deficiency anaemia.
ii. Copper and Manganese:
Help in the conversion of iron into haemoglobin by catalytic action, and
iii. Cobalt as a component of vitamin B12. It is of proved value in man and lower animals. The nature of action probably same as Mn and Cu.
iv. Calcium:
Helps indirectly by conserving more iron and its subsequent assimilation.
2. Bile Salts:
Presence of bile salts in the intestine is essential for the proper absorption of these metals.
3. Endocrine Glands:
i. Thyroxine is of Undoubted Value:
Hypothyroidism is associated with hypochromic, macrocytic anaemia due to lowered metabolic activity in bone marrow,
ii. Adrenal Cortex:
Adrenocortical insufficiency is often associated with anaemia; polycythemia might be present in Cushing’s syndrome. The changes are possibly due to general metabolic alterations and not any direct effect on bone marrow.
4. Vitamins:
Vitamins C, B6, and B12, folic acid, riboflavin, pantothenic acid and nicotinic acid are all important.
5. Pigments:
i. Bile Pigments.
ii. Chlorophyll and other Porphyrins
Deficiency of these factors will give rise to less haemoglobin formation and therefore Hypochromic anaemia. Their mode of action is unknown.
Late Normoblast:
The same factors that operate in the previous stage are also acting here. But the exact nature of the forces that lead to the disappearance of the nucleus is unknown. T
Erythrocyte:
The normal mature red cell.
Essay # 7. Energy Metabolism of Red Blood Corpuscles:
Mature red cells lack nucleus, DNA, RNA and mitochondria. These cells are not capable of synthesising haemoglobin. Krebs cycle is absent. But nucleated R.B.C of bone marrow can be compared with other nucleated tissue cells of the body in respect of its metabolic processes. Mature red cells contain no glycogen and for metabolic processes it has to depend upon plasma glucose that has constant access through erythrocyte membrane.
The exact mechanism of transport of glucose through the membrane is not clearly known. But most of the investigators are of opinion that this is happened mostly through active transport mechanism.
As Krebs cycle (TGA cycle) is absent in mature R.B.C. (non-nucleated), the metabolic breakdown of glucose takes place through:
(a) Embden-Meyerhof glycolytic pathway (vide Metabolism), and
(b) Pentose-phosphate pathway or Hexose monophosphate shunt (vide Metabolism).
Thus the energy requirement of the R.B.C. is obtained from the above two metabolic processes. The longevity of red cells depends upon the maintenance of these energy-producing metabolic processes. The energy is required for the active transport mechanisms of the R.B.C. During active transport, Na is pumped out of the R.B.C and K is pumped in. Besides this, structural integrity and the transport of glucose depends upon the availability of ATP.
Normal life span of R.B.C. is about 120 days. In young R.B.C. the enzymes concerned with the metabolic pathway of glucose breakdown are present in a large amount. With the aging processes of the R.B.C., level of certain enzymes is decreased.
Hexokinase, which acts in the first step of glucose metabolism, is decreased in amount during aging process. Glucose-6-phosphate dehydrogenase, which catalyses the first step of hexose monophosphate shunt is decreased in amount considerably during aging.
Other enzymes which take part in different metabolic processes of R.B.C. are also decreased considerably. With the alteration of glucose metabolism, the ATP source of the R.B.C. is decreased. Due to reduction of available ATP, the structural integrity of the R.B.C. is lost, transport mechanisms are disrupted and ultimately destruction of the cells occurs.
Essay # 8. Life Span of Red Blood Corpuscles:
The average span of life of a mature red blood corpuscle was formerly believed to be about 3-4 weeks. But recent experiments, (using radioaetive Fe, or glycine labeled with isotopic 15N, which enters into the composition of haemoglobin) indicate that it is about 120 days in man.
Essay # 9. Fate of the Red Blood Corpuscles:
As the cells grow senile they change their shape and size and become more brittle. At first the cells throw out processes like pseudopodia and become flask-shaped. These are called poikilocytes. These processes are broken off and in this way the R.B.C disintegrates. This fragmentation takes place in the circulation and the fragments are swallowed up by the R. E. cells.
The R. E. cells of the spleen, liver, etc., can also engulf the senile red blood corpuscles as a whole and break them down intracellularly. Haemoglobin is released and by degradation opening of the porphyrin ring system occurs. The degraded compound is known as verdohaemoglo- bin or choleglobin where the four pyrrole nuclei form a chain instead of a ring. In the next stage it is broken down into protein and haem.
Fig. 4.7 Protein is broken down into amino acids. The iron present in the haem is stored in the body as ferritin and haemosiderin which help in the formation of new haemoglobin. The rest of the haem molecule is converted into a yellow pigment bilirubin which is oxidised into a green pigment biliverdin or according to some, biliverdin is first formed and which by reduction forms bilirubin.
Bilirubin and biliverdin probably combine with plasma α1-globulin and circulate through the blood stream and enter the liver. In the liver cells bilirubin and biliverdin are separated from globulin and conjugate with uridine diphosphate glucuronate to produce monobilirubin and dibilirubin glucuronide (cholebilirubin), the uridine diphosphate is set free.
These compounds enter the duodenum through the bile duct and then into the intestine. In the large intestine by bacterial action they are changed into stercobilinogen (urobilinogen). Some urobilinogen is reabsorbed and excreted in the urine as urobilinogen. The rest is excreted in the faeces as stercobilinogen and stercobilin which are responsible for the brown colour of the stool (Fig. 4.7).
Essay # 10. Red Blood Corpuscles and Haemoglobin:
i. Colour Index: (C.I):
It is calculated as follows:
The haemoglobin percentage is determined as well as the red cell count. A count of 5 million red cells per cu mm is taken as 100%. If a subject is found to possess 60% haemoglobin and only 4 million red cells (i.e., 80% of the normal 5 million), then the colour index will be 60/80 = 0.75.
The normal colour index is 1, but slightly lower index, i.e., 0.8, is more commonly found and is also not abnormal. Colour index indicates the proportion of haemoglobin present in each red cell with respect to normal. In Hypochromic anaemia the index is low, in Hyperchromic or Macrocytic anaemia the index is above 1.
ii. Mean Corpuscular Diameter (M.C.D.):
In film preparations the average diameter is 7.2—7.3 µ.
iii. Mean Corpuscular Volume (M.C.V.):
Anaemia is macrocytic when the M.C.V. is over 96 cu. µ, microcytic if below 85 cu. µ and normocytic at normal level of it.
iv. Mean Corpuscular Thickness:
(M.C.T). = 2.1 – 2.2 µ.
Mean corpuscular thickness can be determined form the formula:
v. Volume Index:
= Normal Average 1 (Range 0.85-1.5).
vi. Relative Volume of Packed Red cells and Plasma:
This is determined by centrifuging oxalated blood in Haematocrit. The cells occupy 45% of the total volume and the rest is made up by plasma. The ratio, cell; plasma = 45: 55 is a good normal.
vii. Mean Corpuscular Haemoglobin (M.C.H.):
It is calculated as follows:
The average figure is 29.5γγ ± 2.5. [1micro-microgram or γγ is 1 million-millionth of one gram.] This indicates the absolute amount of haemoglobin per cell, e.g., each red cell contains, on the average 29.5 γ of Hb.
viii. Mean Corpuscular Haemoglobin concentration (M.C.H.C.):
It is calculated in the following way:
This figure indicates how much of the average volume of the corpuscle is filled up by haemoglobin. Normally 35% of the average volume of each red cell is filed up with Hb. If the M.C.H.C. is below 30% the anaemia resulting is hypochromic type. Value is never higher than above normal. Anaemia with normal M.C.H.C. is normochromic.
ix. Saturation Index:
It indicates the concentration of haemoglobin in the red cell and is obtained by:
